PROJECT SUMMARY Human noroviruses are a significant cause of epidemic and sporadic gastroenteritis around the world. They are the leading cause of severe childhood diarrhea in the United States as well as a major cause of severe childhood diarrhea in developing nations. Despite the global impact of this pathogen, host factors involved in viral infection and disease are not well understood. Recently, we discovered that commensal bacteria aid human norovirus infection of cultured B cells and murine norovirus infection in vivo. In fact, exploitation of commensal bacteria is emerging as a common mechanism among several enteric viral pathogens. These findings provide a fundamentally important clue to understanding human norovirus pathogenesis, and we speculate that interactions with bacteria not only impact the virus but influence the commensal gut flora as well. The response of these bacteria to external stimuli play a crucial role in a variety of biological processes including regulation of intestinal permeability and modulation of host immune responses. Therefore, understanding the effect of viral interactions on these commensal bacteria is critical for fully elucidating mechanisms of viral infection. We hypothesize that HuNoV interactions with bacteria alter bacterial gene and protein expression, ultimately leading to changes in bacterial activity. Supporting this hypothesis, our preliminary RNA-seq data uncovered several bacterial genes that are differentially regulated in the presence of human norovirus virus-like particles. Thus, the objectives of our proposed research are to fully examine the impact of human norovirus-bacterial interactions on gene and protein expression of select commensal bacteria using RNA-seq and proteomic analysis (Specific Aim 1) and to determine if this interaction results in changes in bacterial activity (Specific Aim 2). Results from these studies will provide critical insight into the impact of enteric virus interactions on commensal bacteria and provide a greater understanding of the mechanisms of infection used by human noroviruses.